Photonic entanglement is the key resource in quantum technologies such as quantum communication, quantum computing, therefore to engineer the entangled sources especially with controllability, high-efficiency, stability and portability has always been a popular topic in the quantum community. Traditionally the ways to generate entangled photons include: (1) the atomic transition [1]; (2) four-wave-mixing (FWM) in the atomic ensembles [2]; (3) FWM in silicon, silica-on-silicon [3,4] or fibers [5,6]; (4) Spontaneously parametric down conversion (SPDC) processes in quadratic nonlinear crystals [7,8]. The first way based on the cascaded atomic transition was adopted in the early stage of quantum optical experiments and eventually was abandoned since the generated two-photon state is not perfect. Nowadays people usually adopt the other three ways. Among them, the most commonly used way is the quadratic process in nonlinear optical crystals since it is involved with high photon-flux and simple setups. In the quadratic nonlinear crystals, a pump photon will split into a pair of down converted photons, namely the signal and idler or the entangled photon pair with lower frequencies. During this process, the energy conservation and momentum conservation must be satisfied. There are two types of momentum conversation or phase-matching ways according to different types of nonlinear crystals. One is the homogeneous birefringence phase-matching (BPM) crystal and the other one is the optical superlattice or quadratic nonlinear photonic crystal (NPC). The NPC is an artificial nonlinear optical crystalwhich is fabricated based by the domain-engineering technique in the ferroelectric crystals like lithium niobate (LN) or lithium tantalate (LT). Due to the large nonlinear coefficient being used, it can produce entangled photons with high-photon flux at desirable wavelength [9-12]. Particularly the SPDC efficiency can be further enhanced when the NPC is fabricated into waveguides [13,14].
However, from the BPM crystal, NPC and waveguide NPC, usually only one single entangled source can be generated. In addition a lot of external optical elements should be used to manipulate the entangled photons including interfering, filtering, collecting etc. which will bring challenges to the size, stability and scalability of the entangled sources.